Ceramics have been long used as the substrate for making tiles that may be used in a variety of environments, including wall tiles and floor tiles. The ceramic material composition, its method of manufacture, and its physical and mechanical properties typically will vary depending on the environment and circumstances in which the tile is employed.
Ceramic tiles conventionally have been manufactured by firing a clay material in a kiln to vitrify the clay. Ceramic tiles often are glazed to provide water, frost, chemical and abrasion resistance. Glaze compositions typically are applied to prefired clay which has been subjected to a bisquing process designed to remove moisture from the clay and solidify it somewhat. At the point of firing, the glazes become vitrified and inert, fusing with the ceramic surface to form a substantially single molecular structure.
Although floor tile and wall tile often share some characteristics, such as resistance to water, moisture, and mild chemical agents, they typically differ fundamentally in important characteristics. For example, wall tile may typically be more porous (which permits better adherence to wall substrates) and much less resistant to abrasion than floor tile. In particular, floor tiles need to resist abrasion due to traffic or wheeling of loads and have adequate resistance to skidding, that is, to meet certain coefficient of friction (COF) conditions. In contrast, wall tiles are not required to withstand excessive impact and are not normally subject to freezing and th awing cycles, unless used in an outdoor environment.
Wall tiles may be prepared by firing clay or a mixture of clay and other ceramic materials to produce a relatively thin body that may be either glazed or unglazed. The glaze is a fused facial finish composed of ceramic materials that is usually a vitreous covering that is practically impervious after firing. In some cases, the glaze may be non-vitreous or semi-vitreous. Single-fired glazed tiles are tiles that are glazed before firing. Double-fired glazed tiles may be obtained by firing an unglazed tile substrate, coating the fired tile with glaze and then firing a second time. Double-fired tiles are generally superior in terms of glaze quality and finish. The properties of the glaze composition itself provide the surface sealing, water resistance, etc. To decorate the wall tile, the glaze can be pigmented prior to firing so that the pigmented, fused glaze resulting from the second firing is incorporated onto the surface appearance of the tile.
Alternatively, glazed ceramic tiles can be decorated indirectly through the use of a ceramic decal. Ceramic decals for wall tiles are produced using an overglaze index of colors, that is, a "palette", independently from the actual initial glazing of the tile. To make the decal, an overglaze palette is used in the screening an image onto decal paper for later transfer to the vitrified glazed wall tile. The overglaze is fired onto the tile upon a second firing of the tile. The term overglaze refers to the characteristic of the overglaze to actually sit "on top of" or "over" the substrate glaze.
Overglazing of ceramic wall tiles may be accomplished under low-fire conditions, for example, at temperatures as low as 750-850.degree. C. Such overglazed ceramic tile should resist the normal demands of wall tile, such as resistance to water, moisture, cleaning agents, mild acids, and fading through ultraviolet light exposure is concerned. However, such tiles will not adequately resist the demands of traffic abrasion experienced in floor applications. Since floor traffic would quickly abrade the surface and deteriorate the finish of such overglazed tiles, they are unsuitable for use as floor tiles. Overglaze colors are designed to be produced using specific firing conditions. In particular, in order for the actual colors to mature properly they must be prepared using exact firing temperatures. Secondly, the characteristic to rest above or "over" the glaze, tends to make overglazes prime candidates for surface abrasion if introduced into a floor context. By contrast, in glaze colors sink "into" the substrate glaze by creating a union with the glaze itself. Once they are fired into the glaze, the in glaze colors are enhanced by the hardness of floor tile glazes which are much harder due to their formulation.
Floor tiles conventionally have been made by employing a high-fire glaze. That is, after subjecting the clay to a bisquing process to remove moisture, a high-fire glaze composition is applied to the tile surface, and the tile may then be fired in a kiln at temperatures of about 1000-1280.degree. C. for about 45 to about 120 minutes. The actual firing temperature and duration of firing may vary depending on the unique compositions that are specific to each manufacturer. The glaze compositions are designed to withstand such high-firing conditions so as to render a vitrified product after a single firing. The high-firing conditions create a product having the increased resistance to abrasion required for floor tile applications.
Ceramic floor tile conventionally has been decorated using a direct screen method in which the screened material is applied directly to the unfired, glazed clay bisque. In direct screen printing, an image is generally produced using a coarse line screen of about 65 to 85 wherein a decorative stencil is placed over the glazed clay bisque. Ceramic dyes/inks or colored glaze are then applied through the open parts of the screen to the clay using a roller or squeegee. A first firing is then carried out to glaze the decorated clay.
A significant limitation of the direct screen process is that it cannot be wrapped around contoured surfaces but is limited to screening images only on flat surfaces. Conventional ceramic floor tile substrates are "cushioned", that is, provided with rounded edges. Thus, a direct screen decoration of floor tiles cannot fully decorate the entire tile surface since screening does not wrap around the cushioned edges. This is a significant disadvantage since when a plurality of floor tiles are joined to create a floor surface, there will be a discontinuity in the decorative pattern around the edge of each tile. Thus, a true full field image on the floor is not feasible with conventional ceramic floor tiles. In addition, a decal would be unsuitable for application to the clay bisque since a water-soluble decal cannot be attached to a water-soluble pre-fired glaze without the decal losing its definition. Also the decal would destroy the integrity of the unfired glaze. The unfired glaze tile is highly fragile and easily contaminated and damaged upon handling. In addition, the decal cannot be positioned on the tile properly in such a situation.